1. Field of the Invention
The present invention relates to a treating agent used for desulfurizing molten steel in a reaction vessel having a lining of MgO-containing basic refractory, and to a method for desulfurizing the molten steel in such a vessel. The present invention also relates to a method for decreasing inclusions in the molten steel. The present invention proposes a treating agent and desulfurizing method which attains an effective desulfurization while preventing erosion of the reaction vessel. The present invention also proposes an effective method for decreasing the above-mentioned inclusions.
2. Description of the Related Art
Recently, strict requirements are demanded for the requisite properties of steel materials. Such strict demands are particularly concentrated upon the pipe line materials, which must have anti-hydrogen-induced cracking characteristics, and upon the materials used for ocean structures, which must have anti-lamelar tear characteristics. To meet such demands, it is essential to reduce the content of sulfur (hereinafter referred to as "S") in molten steel as low a level as possible. In addition, gaseous components, i.e., nitrogen (hereinafter referred to as "N") and hydrogen (hereinafter referred to as "H") as well as oxide-based inclusions must be decreased.
Desulfurizing methods are roughly classified into those for desulfurizing molten pig iron and those for desulfurizing molten steel. The former is performed during treatment of the molten pig iron, and the latter is performed during or after refining the molten pig iron into molten steel. In order to melt steels having an ultra-low S concentration, these methods must be combined.
It is common practice in the desulfurization of molten steel to inject a mixture of CaO with other components, or a Ca alloy into the molten steel, contained in a ladle, with the aid of a carrier gas. This is referred to as the injection method and a feature of this desulfurizing method resides in utilizing an intense reaction between the slag and the molten steel brought about by an intense stirring thereof due to the injection. The desulfurization method which is based on the reaction between the slag and the molten steel under intense stirring, contains drawbacks in that rephosphorization from the ladle slag is likely to occur during the treatment, and further, that the recovery rate of the alloying components, such as Al alloy, contained in the desulfurizing agent becomes low. Furthermore, the temperature fall of molten steel during the treatment is relatively great, and non-agitation of the bath level surface cannot be maintained due to the intense stirring; with the result that the molten steel inevitably absorbs the gas components, such as H, N, and the like, from the ambient air or slag. Therefore, when low nitrogen steels or steels required to have both low H and N contents, as in steels for use as plates, are to be melted, the steels subjected to the injection method must be again subjected to a degassing process, such as an RH or DH process. The additional process such as an RH or DH process leads to a further temperature fall of the molten steel. The molten steel must therefore be super-heated in a converter or the like, to compensate for the temperature fall during the additional process. Thus, an increase in the treating time of molten steel in a converter or the like inevitably occurs due to the super heating. In addition, the qualities of the molten steel are frequently unfavorably influenced by the super heating.
In order to eliminate the drawbacks as described above, a simultaneous degassing and desulfurizing method has been recently developed, according to which method a desulfurizing agent is injected together with the carrier gas into the upward stream of the molten steel within a vacuum vessel. Among the proposed methods, the method proposed by the present applicant in Japanese Unexamined Patent Publication No. 60-59011 discloses a method of producing with a small unit weight of the treating agent, molten steels having an ultra low sulfur content and having low N, O and H contents. In this method, a desulfurizing agent is added to the molten steel and a predominant desulfurizing reaction is completed in the steel bath having slag thereon, while the slag is essentially not stirred or caused to fluidize due to the addition of the desulfurizing agent. The above-mentioned molten steels having an ultra low sulfur content and low N, O, and H can be produced by using a treating agent containing at least 20%, preferably 40%, by weight of CaF.sub.2, and containing CaO as the remaining main component.
The present inventors carried out a further study of the method disclosed in Japanese Unexamined Patent Publication No. 60-59011 and discovered the following problems involved therein.
(1) The desulfurizing agent used in Japanese Unexamined Patent Publication No. 60-59011 has a high desulfurization ability, but due to the high CaF.sub.2 content of 20% by weight or more, promotes the erosion of the MgO-containing basic refractories generally used for the reaction vessel for molten steel. Such refractories are formed by using magnesia, magnesia-carbon, magnesia-chrome, or dolomite, spinel, alone or in mixture. Accordingly, when ultra low sulfur steels are frequently treated in such a refining vessel, the life of the vessel is shortened and the refractory cost is increased.
(2) When the desulfurization method disclosed in Japanese Unexamined Patent Publication No. 60-59011 is applied for molten steels having a relatively high quantity of inclusions, the treating agent injected into the molten steel and the oxide-based inclusions of molten steel coagulate and incorporate with one another, with the result that Al.sub.2 O.sub.3 and the like are absorbed into the treating agent. Since the desulfurization ability is lessened due to such absorption, the treating agent containing a high CaF.sub.2 concentration must be injected at a high unit weight per quantity of molten steel, when ultra-low steels having S content &lt;5 ppm are to be obtained. This also leads to a shortening of the life of the reaction vessel and to an increase in the refractory cost.
As described above, according to the method of this publication, the slag on the bath level essentially is not stirred or fluidized, and therefore, engulfing of slag in the ladle and ambient air into the molten steel can be advantageously suppressed. Nevertheless, where the treating agent having a high CaF.sub.2 concentration is injected at a great quantity so as to produce the ultra-low sulfur steels, the treating agent having a high CaF.sub.2 concentration accumulates beneath the ladle slag, so that the melting point of the slag is considerably lowered, and hence the slag becomes completely molten up to the top surface thereof. In this case, the oxygen in the entire slag is easily moved, and hence the oxygen is liable to pass from the slag into the molten steel. Furthermore, when the casting process is carried out after the treating process, the slag as mentioned above is liable to be engulfed in the molten steel due to a swirl formed in the pouring stream when pouring into a tundish or a mold, and cause the inclusions to form.
Accordingly, a novel process which attains both an effective desulfurization and a decrease in the inclusions, must be developed.